It is well-known that the metals or alloys commonly used in technological applications, both the types used in ancient times, and those used nowadays, undergo, over time, structural modifications and/or more or less deep and radical chemical or mineralogical transformation processes and corrosion processes. The alterations and transformations of metal artefacts may vary from modifications concerning the superficial layers, and internal microstructural modifications which concern the granulometry of the microcrystals which constitute metals and alloys, up to the most serious decay, so extended as to render unrecognizable the original metal or alloy.
For example, copper and its alloys, when exposed to air rich in moisture and carbon dioxide, sulfur dioxide and hydrogen sulfide, are coated by thin layers of oxides, salts and basic salt hydrates, whilst iron artefacts, in particular if recovered by excavations, undergo nearly always extremely extended and devastating alterating processes based on corrosion.
In general, the restoration of metal materials, in particular those of archeological and historical-artistical interesting, involves the following operations:
1) Cleaning and possible removal of surface corrosion products; PA1 2) Reassembly; PA1 3) Integration; PA1 4) Aggregation; PA1 5) Protection.
Obviously, all, or some, of the above operations from (2) to (4) can be avoided if the artefact is in such conditions as to only require the protection operation.
As regards the two last operations, which are the subject-matter of the present invention, and can be accomplished in one single step, a plurality of essential characteristics are required.
In order to be able to perform the desired action, without simultaneously inducing side decay phenomena and, anyway, undesired side effects, the aggregating-protecting agent should display the following characteristics:
Adhesiveness:
Need for a good adhesion to the underlying substrate, with the guarantee that no peeling or delamination possibilities exist;
Reversibility:
Possibility of removal of the surface film with solvents which do not have an influence on the artefact, or on the material used for bonding or reintegrating the material;
Elasticity:
Also in case of dimensional changes of metal support, owing to temperature changes, the aggregating-protecting agent should retain its continuity, and consequently also its functionality;
Unalterability and stability:
Absence of changes in colour and chemical and physical characteristics over time, also in a polluted atmosphere; absence of chemical interaction between the aggregating-protecting agent and the substrate;
Inertness towards moisture:
The product should not absorb water and anyway should prevent the substrate from coming into contact with water; it should be neither degraded, nor crosslinked;
Insensibility to U.V. light:
Exposure to light, in particular to sun light, should not cause phenomena of degradation of the protectant;
Transparence:
The protectant should not alter the original appearance of the underlying material, and allow a perfect view of the artefact;
Insensibility to heat and temperature excursions:
Possible exposure to heat sources should not alter the functionality of the aggregating-protecting agent, nor phenomena of low-temperature embrittlement should occur; for indicative purposes, the product should retain its functionality at least within the temperature range of from -25.degree. C. up to +100.degree. C.;
Easiness of application:
Above all for delicate pieces, which should not be submitted to treatments which may be dangerous for their integrity.
It is well-known that in order to protect and reaggregate artefacts constituted by metals or metal alloys, organic substances are frequently used, such as waxes, of the type of beeswax, which contain functional groups of ester, carboxy, alcohol types.
Microcrystalline waxes based on saturated hydrocarbons, acrylic resins and silicone resins are used as well; the three last classes of compounds are those which are presently most widely used in the concerned sector.
As a matter of fact, inasmuch as they contain functional groups of carbonylic type, waxes, such as beeswax, show poor stability to U.V. light, show a tendency to discolour with time (i.e., to turn into yellow), and their use on artefacts exposed to atmospheric agents requires frequent maintenance operations in order to obviate their insufficient stability.
Although they are more stable to the action of U.V. light, microcrystalline waxes, owing to the presence in their molecule of tertiary hydrogen atoms (C--H) undergo the photo-oxidative attach leading, as in the above case, to the formation of acidic products, or to products anyway harmful to the substrate.
Furthermore, the low melting or softening point, close to 63.degree. C. for beeswax and 70.degree. C. for microcrystalline waxes, may lead to these protectants leaving the substrate by flow.
As regards acrylic resins, their tendency to undergo chemical alterations due to natural long-term ageing was already demonstrated (Proceedings of Meeting &lt;&lt;Bononiachem 88&gt;&gt;-National Meeting of Italian Chemical Society, Bologna, 1988, page 272). Also silicone resins show phenomena of instability upon natural ageing, and upon accelerated ageing when submitted to U.V. light (Proceedings of Meeting &lt;&lt;Science e Beni Culturali [Sciences and Cultural Patrimonyl&gt;&gt;, Bressanone, 1986, pages 597-611); in both cases, FT-IR analysis demonstrated, after ageing, the presence of bands which might be correlated with the formation of acidic moieties.
Because--as illustrated hereinabove--the presently used product to not display all the characteristics required from an aggregating-protecting agent for metal surfaces, the need emerged for developing suitable compounds endowed with higher chemical stability.